1. Field of the Invention
The present invention relates to a power steering system for vehicles, and more particularly, to an electric power steering system for vehicles which produces auxiliary steering torque by means of a steering servo device using an electric motor.
2. Description of Relevant Art
In recent years, in view of problems in hydraulic type power steering systems such as that the structure thereof is complicated, a variety of electric type power steering systems for vehicles have been proposed.
As one of such electric power steering systems for vehicles, there is disclosed an example of an analogue control type in UK patent application Publication No. GB 2132950 A, published July 18, 1984.
The electric power steering system for vehicles according to this UK Publication includes an input shaft as a steering shaft connected to a steering wheel, an output shaft interconnected through a universal joint with the input shaft and through a rack and pinion type gear with a tie rod of steered wheels, an electric motor for supplying auxiliary torque through a reduction gear to the output shaft, a torque detecting mechanism disposed on the input shaft to detect steering torque acting on the input shaft, a motor drive circuit for driving the electric motor, and an analogue type control circuit for feeding the motor drive circuit with a control signal in accordance with a detection signal from the torque detecting mechanism.
The analogue type control circuit is adapted to have an armature voltage PWM-controlled to be impressed to the electric motor, with such a polarity that causes the motor to rotate, during a steering operation in either direction of rotation of the steering wheel, in a direction corresponding to the steering direction; there being fed back a signal corresponding to an armature current. And, auxiliary torque produced by the electric motor is supplied through the reduction gear to the output shaft so that it is contrived to render the steering force light.
However, not limiting to the case of the power steering system according to the UK Publication, in various electric power steering systems proposed in recent years, their steering servo devices have friction elements such as an electric motor and a reduction gear. Moreover, the armature voltage to be impressed to the electric motor in dependence on steering torque is given as a function of the load from the road surface side only. Therefore, in a case where an operation of the steering wheel from its neutral position to the left or right is started at a low speed with a relatively small steering force, the armature voltage becomes small and there appears a steering torque region in which no auxiliary torque that corresponds to the steering load due to friction elements of the system is produced. In such torque region, it is needed to rotate the friction elements, i.e. the electric motor and the like, from the side of the steering wheel. As a result, the steering operation in starting turning the steering wheel feels relatively heavy, with the possibility of degrading the steering feeling.
Incidentally, between the armature voltage Va and armature current Ia of the electric motor, there is such a relation that Va=Ia.multidot.Ra+K.multidot.Nm, where Ra is the internal resistance of the motor, Nm is the speed of rotation of the motor, and K is a constant in terms of induced electromotive force of the motor. Moreover, the rotation speed Nm of the motor is proportional to the steering speed of the steering wheel. However, even if the armature voltage Va to be impressed to the electric motor were controlled taking into account an induced voltage Vi (Vi=K.multidot.Nm) of the motor, such problems as described could not be overcome. In other words, even if the induced voltage Vi of the motor that is proportional to the steering speed of the steering wheel were taken into account, in addition to the road surface side load, to control the armature voltage Va, the above problems could not be overcome. This is because a duty value attributable to the induced voltage term K.Nm of the armature voltage Va becomes small when the steering operation from the neutral position of the steering wheel is started at a low speed.
On the other hand, in recent years, due to the advantage that complicated control functions can be achieved with a relatively simple system constitution, microcomputer systems fundamentally adapted for digital signal processes tend to be utilized as control means of various working devices.
In this respect, it would be desirable to utilize a microcomputer system in the form of a control device for such steering systems as described.
However, since in general microcomputer systems are unable to concurrently read many input signals and besides are adapted to execute signal processes thereof in a sequential manner depending on a system clock pulse, in attempts to utilize a microcomputer system to achieve control functions similar to those of an analogue control circuit of a conventional electric power steering system of the type described, for example, there is an attendant restriction or problem such that a predetermined process time is required.
In this respect, in cases where a feedback control is performed by using a microcomputer system, it is needed to repeat a feedback loop many times and such a problem as described above becomes significant.
For such reasons, in cases where a microcomputer system is utilized as a control device of an electric power steering system, there is an attendant possibility such that an electric motor for producing auxiliary torque can not be so controlled as to sufficiently follow up the speed of steering operation, thus resulting in the possibility of unsuccessful realization of optimum steering feeling.
In view of such a problem in conventional electric power steering systems as described, the present invention has been achieved to effectively solve same. Particularly, the invention has been achieved to overcome the aforementioned restriction or problem attending thereon even in cases where for the solution of such problem a microcomputer system is utilized as a control device.